This invention relates generally to hermetically sealed, electromotively driven compressors for air conditioning and refrigeration applications and the like and more particularly to protective devices used with such compressors.
Protective devices for de-energizing hermetically sealed, electromotively driven compressors which are mounted on a sealed terminal assembly inside the sealed casing of the compressors and which have a fuse function actuated by detecting an over-current generated by a faulty motor or the like are known.
FIG. 14 is a perspective view of the back side of a prior art protective device 101 for a hermetically sealed type electromotively driven compressor. Surface 103 of housing 102 of protective device 101 has holes 104 that correspond to electrically conductive pins of a sealed terminal assembly of the compressor casing (not shown in the drawing). The protective device has some electrically conductive parts that are exposed or openings which expose conductive parts. For example, the electrically conductive part of a protector 106 is exposed at opening 105 that was formed in connection with the forming of housing 102. In addition, electrically conductive parts such as a current fuse 107 and that part to which the fuse is welded are also exposed.
It is known to maintain an insulation distance for these exposed parts as stipulated in an official standard for electrically conductive parts of the sealed terminal or other conductive parts in a hermetically sealed casing.
However, if a large electric current, generated at the time of a fault condition of the electric motor, is detected and the current fuse is melted, an extremely large reverse electromotive force is generated at that moment. This reverse electromotive force generates an electric discharge phenomenon. If an electrically conductive part is exposed on surface 103 that faces the sealed terminal in the casing and unless a sufficient insulation distance is provided, the electric discharge phenomenon between the electric current fuse can jump to the sealed terminal or conductive parts in the sealed casing, thereby forming an electric circuit which could develop into such problems as a sealed terminal jump or earth leakage due to internal short circuiting.
In the conventional protective device for a hermetically sealed electromotively driven compressor, it has been difficult to avoid exposing the electrically conductive parts due to restrictions on the techniques used for the preparation of the electrically conductive parts or for the preparation of the protector assembly. The fuse function that is actuated by detecting a large electric current generated at the time of a fault of the electric motor is a comparatively new technology and using the standard for insulation distances based on the official specifications that presently exist does not adequately deal with this new technology. The development of sealed terminal jumps due to internal short circuiting or an earth leakage have been reported in the past despite the fact that these standards were observed.
Leakage of a coolant gas in a hermetically sealed electromotively driven compressor and equipment provided therewith is an example of an abnormal condition to which such compressors can be subjected and a room air conditioner is a representative device employing such a hermetically sealed electromotively driven compressor. In the case of the normal room air conditioner, the condenser is usually referred to as outdoor equipment and the evaporator as indoor equipment and these are arranged at a distance from each other. If there is some fault in the installation work of the equipment, a crevice or cracking could occur in the cooling system which should be air-tight, with a result that coolant gas starts leaking and air starts entering the system. Even in the case of a refrigerator and the like for which no piping installation work is required, such a crevice or cracking could develop if there is damage during the course of transportation or usage, or if there is a defect in manufacturing, with a similar result of coolant gas leakage, thereby allowing air to enter into the cooling system.
Coolant gas leakage is one of the major causes for burning of electric motors in such systems. In view of the fact that the coolant gas also serves to cool the motor by removing heat produced by the motor when the gas circulates inside the hermetically sealed electromotively driven compressor, a leak of the coolant gas brings about a rise in the temperature of the motor. When this happens, the temperature of the motor increases but the operating current decreases. Therefore, protection cannot be provided by a protector of the type that detects an over-current for shutting off the electric current. As a consequence, the electric motor is easily damaged. If the electric motor in a hermetically sealed electromotively driven compressor is damaged, the insulation film on the windings is destroyed, thereby developing short-circuiting which will, in turn, bring about the generation of an extremely large electric current. If such a large electric current is allowed to continue, the sealed terminal pins of the hermetically sealed electromotively driven compressor can be blown out of the terminal assembly or a fire can result due to over-heating by the electric current. In order to solve such a problem, protector devices have been provided with an electric current fuse for the purpose of shutting off such a large electric current. On the other hand, however, there are cases where the sealed terminal jump or earth leakage of the hermetically sealed type electromotive compressor occurs through the use of the electric current fuse. That is, as stated above, when the large electric current generated upon burning of the electric motor is shut off, an extremely large reverse electromotive force is generated in the motor.
A rough schematic of the electric circuit of the normal operation of equipment provided with a hermetically sealed electromotively driven compressor is shown in FIG. 15. As shown in the figure, an electric current fuse 118d is connected in series with the power source circuit of the electric motor having a main coil 118a, a start coil 118b and a capacitor 118c. Due to voltage from the power source, electric current flows driving the motor and the hermetically sealed compressor is operated normally and, in this state, electric current fuse 118d is not affected.
FIG. 16 is an electric circuit schematic at the moment when a fault develops in the equipment that is provided with a hermetically sealed electromotively driven compressor, causing a large electric current and actuation of the current fuse. In this state, fuse 118d is melted and the power source circuit is open as shown in the figure. At this instant, a reverse electromotive force 119a is generated in the direction of continued current flow in conformity with Lenz""s law. This reverse electromotive force 119a, which is dependent upon the size of the motor and the kind of the electromotive system, sometimes reaches a range between approximately 6000 and 9000 volts because the electric current fuse cuts off the current in an extremely short period of time. This is clearly observed from the function of the electromagnetic induction:
Electromotive Force e=M(di/dt) 
(M=mutual inductance, di=amount of a change in the electric current, and dt=changed time)
In the case of a commercial power source, one phase is usually grounded. Schematics of the electric circuit in equipment which is provided with a hermetically sealed electromotively driven compressor including its grounding is shown in FIGS. 17 and 18. In FIG. 17, the commercial power source is grounded at 120a on the side where the electrical current fuse 118d is connected. In addition, a dashed line indicates a sealed casing 120b grounded at 120c. In FIG. 18, the commercial power source is grounded at 121a on the opposite side of the electric current fuse 118d. Likewise, the dashed line indicates the sealed casing 121b grounded at 121c. 
The state of the circuit at the instant fuse 118d melts due to an over-current generated as a result of a fault in equipment provided with a hermetically sealed electromotively driven compressor is shown in FIG. 19. Along with actuation of fuse 118d, the circuit opens and a reverse electromotive force 119a is generated in the motor as shown in the figure. If, when this happens, the insulation distance 122a between the metal part inside of sealed casing 120a or the metal part at the sealed terminal and the electrically conductive part of the protector assembly is merely the distance according to the official standard, it becomes impossible to withstand the reverse electromotive force that has been generated by the electric motor and the electric discharge will jump over. In view of the fact that the sealed casing 120b is ordinarily grounded at 120c, the electric current due to the electric discharge ends up flowing to the power source through the grounding. This state is shown in FIG. 20. The reverse electromotive force 123a generates an electric current and this electric current flows through the normal circuit 123b and jumps over to the sealed casing 120b from that part where the insulation distance is deficient generally in the neighborhood of the electric current fuse 118d, with a result that an electric discharge 123c takes place. In other words, electric current 123d that has flowed in sealed casing 120b flows into ground 123e through grounding 120c and enters grounding 120a of the commercial power source from the ground as shown at 123f, thereby forming a complete electric circuit. Once the electric circuit is formed by an electric discharge, the ambient atmosphere is ionized, with the electric discharge being continued even with the low voltage of the commercial power source in some cases. As a consequence of this, continuous electric conductance takes place with the power source voltage along such a route as shown in FIG. 21.
In addition to the above, when a leakage of the coolant gas takes place, air enters from outside and the pressure inside the hermetically sealed type electromotively driven compressor becomes approximately equal to the atmospheric pressure. As a result, the insulation resistance suddenly decreases, thereby making it even easier for the above described electric discharge phenomenon to take place. According to Pachen""s rule as described in Electricity and Magnetism in 1.1.11 in Chapter 1, dealing with electricity, in the Revised Edition Six of Mechanical Engineering Handbook, Third Print, Sixth Edition, revised on Mar. 20, 1982 by the Society of Machinery of Japan, for example, the minimum voltage for the development of an electric discharge between a plane electrode and an edge electrode in ordinary air is 1000 volts for one millimeter of insulation distance.
Official standards such as IEC Standard 60730-2-4 stipulate that the spatial distance relative to the motor protector inside compressors whose ratings are less than two kw and less than 300V is to be greater than 1.6 mm. Along this line of thinking, it can be stated that the insulation pressure resistance at a time when a leakage of the coolant gas occurs and air starts coming in decreases to the vicinity of 1600V.
1000 (V/mm)xc3x971.6 (mm)=1600V
The insulation distance as stipulated by such an official standard cannot be termed sufficient as applied to such an abnormal situation as when an electric current fuse is actuated.
It is an object of the present invention to provide a solution to the above noted problems of conventional technology. Another object of the invention is the provision of a protective device for use with hermetic type electromotively driven compressors without causing pin blow out or earth leakage even when a fuse is blown out by huge currents generated in abnormal situations such as a gas leak.
A protective device made in accordance with the invention is particularly adapted for use with a hermetically sealed type electromotively driven compressor. The protector device has a current fuse that actuates upon detecting a predetermined over-current provided therein and is provided with an insulation member so that the insulation distance of any electrically conductive part in the protective device, including the fuse, has a value which is greater than a predetermined value of 9.5 mm or greater as measured along the surface. By maintaining an insulation distance at 9.5 mm or greater, an insulation pressure resistance of 9500 V can be achieved according to the following equation:
1000 (V/mm)xc3x979.5 (mm)=9500V
Even in the event of coolant gas leakage, with a consequential leakage of air into the cooling system and a resultant lowering of the insulation pressure resistance, therefore, it becomes possible through the invention to secure adequate insulation pressure resistance against the high voltage that is generated when the electric current fuse is actuated.
According to a feature of the invention, the insulation member is formed in such a way as to cover or block electrically conductive parts on that side of the protective device which faces the external connection terminal assembly in the casing of the compressor.
According to another feature of the invention, the said insulation member also serves as a locking member for locking the lead wire for connective purposes thereby reducing the dimension in the height direction of the housing of the protector device. This is all the more economical as there is no need to increase the number of the parts involved.
According to yet another feature, the said insulation member includes a protruding portion that sticks out from the side of the housing of the protective device, thereby providing sufficient insulation distance and enabling a reduction of the height direction dimension of the housing, making it possible to obtain a stronger electric insulation member. According to a feature of the invention, the housing of the protective device is made of a resin material and said protuberant piece is integrally formed with said housing, thereby making it possible to reduce the number of parts and assembling steps required and, at the same time, making it possible to select the insulation material according to the particular requirements involved.